please help to obtain the next node value

i have the next xml
if i am into the fist file node i can get the fist main node value
but how can i get , from the fist file node the second main node value?
thjanks

<?xml version="1.0" encoding="UTF-8"?>
<root>
<file>
        <main>1</main>
</file>
<file>
        <main>2</main>
</file>
<file>
        <main>3</main>
</file>
</root>